1. Field of the Invention
The present invention relates generally to the field of gas storage systems. More particularly, the present invention relates to gas storage systems that are based on materials that adsorb gas. Specifically, a preferred implementation of the present invention relates to a system that uses a carbon fiber composite molecular sieve (CFCMS) material for gas adsorption. In this implementation, desorption of the previously adsorbed gas is achieved by passing an electrical current through the CFCMS material. The present invention thus relates to a gas storage system of the type that can be termed electrical swing adsorption.
2. Discussion of the Related Art
Historically, natural gas has been an important primary energy source and is one of the cleanest burning fossil fuels. It is becoming increasingly important as a competitor for liquid transportation fuels such as diesel, gasoline and alcohol (e.g., methanol and ethanol). The two disadvantages, in comparison with liquid fuels, most often cited with regard to natural gas are, first, the expense of transporting it from the site of production to its point of use and, second, its low energy density, both per unit mass and per unit volume.
For use as a vehicle fuel, the low energy density disadvantage can be partially overcome by the use of compressed natural gas (CNG). For example, a pressure vessel can be filled with compressed natural gas at pressures as high as 250 atm. Another way to address the low energy density disadvantage issue is by the use of liquified natural gas. However, a problem with the use of either of these techniques is that they both impose considerable additional costs. Thus, there is a need for a storage and delivery system in which equivalent amounts of gas could be stored at moderate pressures (e.g., 20-25 atm) and at moderate temperatures (e.g., 10-30.degree. C.) while still maximizing the amount of natural gas that can delivered to the vehicle from a fuel tank of a given size and weight.
Moreover, another previously recognized problem with the compressed natural gas approach is that all vehicles are subject to mishaps. While gasoline, and to a lesser extent diesel, fuels present a fire and/or detonation hazard in a collision, a high pressure vessel containing compressed natural gas represents a more significant fire and/or detonation hazard. Therefore, there is also a need for a high energy density natural gas storage system that is less hazardous than compressed natural gas.
A previously recognized solution has been the use of granular activated carbons and adsorption/desorption techniques such as pressure swing adsorption and thermal swing adsorption..sup.(1) In pressure swing adsorption (PSA) or temperature swing adsorption (TSA) systems, gases are desorbed by lowering the pressure (or increasing the temperature) of the adsorbent system (thus, the system "swings" from a high to low pressure or a low to high temperature). However, a serious drawback to both of the PSA and TSA techniques is that they impose undesirable conditions on the system. More specifically, the PSA technique imposes the cycling of pressure between adsorption and desorption (i.e., high-low). Similarly, the TSA technique imposes the cycling of temperature between adsorption and desorption (i.e., low-high). Thus, a need exists for a simpler and more effective technique that will result in desorption of most, if not all, of the stored gas without the need for drastic system condition variations.
Moreover, another drawback to the PSA and TSA techniques is that they suffer from an inability to deliver all the adsorbed gas. Complicating factors include the shape of the methane adsorption isotherm at ambient temperatures and effects of adsorption enthalpy. The ideal adsorbent would show complete reversibility, (i.e., high uptake and complete desorption). In practice, most adsorbents cannot be completely desorbed and thus will have a residual gas content. A compromise approach is often taken to try to optimize the ratio of stored-to-deliverable gas. Considerable advantage would be gained by a desorption process which results in delivery of all, or almost all, of the stored gas.
Within this application several publications are referenced by superscripts composed of arabic numerals within parentheses. Full citations for these, and other, publications may be found at the end of the specification immediately preceding the claims. The disclosures of all these publications in their entireties are hereby expressly incorporated by reference into the present application for the purposes of indicating the background of the present invention and illustrating the state of the art.